Ukraine's defence ministry on Thursday said around 20 gunmen seized a Ukrainian warship, the Ternopil, in the port of Sevastopol in Crimea.

"The ship has been taken," Vladislav Seleznyov, the defence ministry's spokesman in Crimea, said on his Facebook page, after earlier telling AFP: "The assault has begun".

On Facebook, Seleznyov said pro-Moscow militiamen and Russian soldiers cordoned off the area, while a boat with the gunmen on board approached the Ternopil and stormed it.

"Stun grenades were used during the assault and automatic fire was heard," he said.

Called by AFP, an officer on board the Slavutich, a second Ukrainian warship next to the Ternopil, did not answer.

Seleznyov did not say whether the Slavutich was also seized by Russian forces.

The commander of Russia's Black Sea Fleet, Alexander Vitko, had reportedly boarded the Slavutich earlier on Thursday in an attempt to negotiate a surrender.

The two warships had been blockaded in the port by the Russian navy and had distanced themselves from the dock in an attempt to prevent an assault.